The use of solvents in various cleaning operations remains widespread, despite increasingly strict environmental regulations concerning the use and/or release into the environment of certain types of solvents. For example, organic solvents (including halogenated solvents) are used extensively for cleaning machined parts during manufacture of the parts. Organic solvents also are used extensively for cleaning parts, such as engine parts and other parts from motor vehicles, during repair or refurbishment operations of such vehicles. In fact, use of an organic solvent is frequently the only practical way in which to remove grease, sludge, varnish, and similar deposits from many types of parts.
Not only are the costs of organic solvents continuing to increase, but certain specific solvents are increasingly the subject of strict governmental regulation in terms of the conditions under which the solvents can be used and in terms of disposal of the solvents. Certain solvents (such as halogenated solvents) have become regulated so intensively that many prior uses of such solvents have become curtailed substantially.
As a result of situations as summarized above, in many instances it is most practical for a user of a cleaning solvent simply to recover and purify (i.e., “recycle”), on site, the solvent that has become laden with solutes as a result of using the solvent for cleaning purposes. In this regard, distillation of the solute-laden solvent mixture is a favored solvent-recycling and -recovery method because distillation can be performed using a compact apparatus that can be installed almost anywhere. As is well known, distillation typically involves heating a liquid solvent-containing mixture in an evaporator vessel (also termed the “pot”) to produce a vapor of substantially only the solvent (disregarding formation of azeotropes). The solvent vapor is conducted to a condenser that converts the vapor into a corresponding liquid. Because mostly solvent vapor is released from the liquid mixture in the pot, substantially all of the solute is left behind in the pot, and the condensed liquid usually is substantially purer with respect to the solvent than the liquid remaining in the pot.
Distillation is well known as a purification technique for any of various solvents, including water. The many types of distillation systems disclosed in the art have various respective features that reportedly improve the utility, efficiency, and/or applicability of the technique for particular conditions of use. Depending upon the particular configuration or condition of use of the distillation system, certain parameters may or may not have substantial importance.
For example, solvent-distillation apparatus are used in automotive repair shops and the like where the removal of grease, oil, sludge, and the like from parts is a critical aspect of the repair business. Advantageously, such solvent-distillation apparatus are compact in size, energy-efficient, safe to use (including under conditions in which personnel are not present), and reliable. An example of such an apparatus is disclosed in U.S. Pat. No. 4,929,312 to the current Applicant. Similar to many other types of distillation apparatus, the apparatus disclosed in the '312 patent includes an evaporation vessel (“pot”) made of metal and configured to contain a solvent liquid to be distilled. The pot includes a heating unit that heats the liquid in the pot sufficiently (typically to boiling) for production of solvent vapor for distillation. The solvent vapor passes from the pot to a condensing unit that cools the vapor to a liquid (comprising purified solvent) that is collected for re-use.
It has been discovered that a key performance criterion for a distillation system, such as the system disclosed in the '312 patent, is an ability to sense the temperature of the liquid in the pot accurately and in real time. In the apparatus of the '312 patent, the heating unit is embedded in a thick mass of metal (e.g., aluminum) at the bottom of the pot. The temperature of the liquid in the pot is sensed, for control purposes, by a thermostatic switch attached to a side wall of the pot or to the mass of metal at the bottom of the pot. With the thermostatic switch at either location, a substantial amount of heat energy must be applied to the metal of the pot to cause a significant change in temperature as sensed by the thermostatic switch. As a result, especially under conditions of temperature change, the temperature sensed by the thermostatic switch does not accurately track in real time the actual temperature of the liquid in the pot. For example, consider a thermostatic switch situated such that the heating unit is between the thermostatic switch and the inside wall of the pot. With this configuration the temperature sensed by the thermostatic switch may lead or not lead, depending upon whether the heating unit is energized or not energized. Also, the resulting dampening of temperature sensing allows substantial swings in temperature of the liquid to occur without being sensed at all by the thermostatic switch. Consequently, safe and efficient operation of the apparatus can be compromised.